As is well known, an electric power steering apparatus mounted in a vehicle such as an automobile produces steering auxiliary power by use of a motor that is supplied with electric power from a vehicle battery, which is a main power source, so as to be driven, and supports a driver's steering power by exerting the steering auxiliary power on the steering axle. In general, the electric power steering apparatus is configured in such a way as to support the foregoing steering power when the vehicle is in an operation mode.
However, in the case where the vehicle battery, which is a main power source, fails in such a way that for some causes, the voltage thereof extremely decreases, the motor of the electric power steering apparatus is not sufficiently supplied with an electric current, whereby stable steering auxiliary power cannot be continuously obtained. Thus, to date, it has been proposed to provide an auxiliary power source having a back-up function for the main power source (for example, refer to Patent Document 1).